The Cascadia subduction zone in the Pacific Northwest of the United States of America is capable of producing magnitude ~9 earthquakes, likely often accompanied by tsunamis. An outstanding question in this region, as in most subduction zones, is the degree and spatial extent of strain accumulation, which will eventually release as an earthquake, on the subduction megathrust. Geodetic observations, including those from Global Navigation Satellite Systems (GNSS), including the Global Positioning System (GPS), may be used to image the strain actively accumulating on a fault before an earthquake ultimately occurs. Technology combining GNSS and underwater acoustic ranging (GNSS-A) is now capable of making centimeter-level horizontal geodetic observations on the seafloor. GNSS-A enables previously inaccessible observations to better image seismogenic portions of the Cascadia subduction zone. Because seafloor geodetic instruments, and the time and logistics associated with observations, can be cost-prohibitive, it is important to identify where deploying seafloor geodetic instruments will provide information that cannot be obtained through a similar investment in onshore geodetic networks. Here we leverage the concept of information entropy to 1) quantify the relative information provided by expanding GNSS observation networks offshore Oregon and Washington and 2) identify optimal locations for a network of seafloor geodetic instruments. The information gained by new observations, and their optimal locations, depends on the expected uncertainties on the seafloor velocity observations, modeling assumptions, and the modeling objectives.
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